Hematopoietic stem cell transplantation (HSCT) is a successful treatment option for many patients with severe hematologic diseases. However, large numbers of transplantable cells are needed and patient survival is compromised when donor hematopoietic stem cell (HSC) numbers are limited. This is the case when umbilical cord blood (CB) is utilized as a donor source for transplantation into adults. Given that CB is readily available, has a lower histocompatibility requirement, and carries a reduced risk of graft vs. host disease, there are clear reasons for using CB for HSCT. To overcome the problem of limited cell numbers, cytokines and growth factors are being utilized to facilitate ex vivo CB expansion pre-transplantation or as therapy post- transplant. The effects of cytokine treatment on stem cell trafficking during transplantation have not been described. Our long-term goals are to understand the mechanisms that govern HSC trafficking and to design innovative methodologies to improve the effectiveness of HSCT. Achieving these goals would have a dramatic impact on outcomes for adult patients undergoing cord blood HSCT. Towards this goal, we recently demonstrated that inhibition or loss of CD26 (DPPIV/dipeptidylpeptidase IV) significantly improved HSCT efficiency. We hypothesize that granulocyte colony-stimulating factor (G-CSF) and granulocyte macrophage-colony stimulating growth factor (GM-CSF) treatment up-regulates the peptidase CD26 resulting in decreased HSCT efficiency and that CD26 inhibition can reverse this negative effect. To investigate this, we propose: Aim 1) to evaluate the in vitro effects of G-CSF, GM-CSF and stem cell factor (SCF;also called steel factor or kit ligand) on human cord blood hematopoietic stem and progenitor cell (HSC/HPC) function. We will do this by evaluating cytokine induced changes in 1a) CD26 RNA expression, CD26 protein expression, and CD26 peptidase activity, and 1b) cell migration, cell adhesion, and progenitor colony formation in the presence or absence of CD26 inhibitors. Aim 2) to evaluate the in vivo effects of G-SCF, GM-CSF, and SCF on human cord blood HSCT efficiency and the use of CD26 inhibition to overcome any detrimental effects that result from cytokine treatment. We will do this by measuring cytokine induced changes in 2a) the homing of human donor HSC/HPC to the bone marrow of NOD/SCID/B2mnull immunodeficient recipient mice 24 hours post transplant, and 2b) the engraftment of short term repopulating cells of myeloid restricted (STRC-M) and dual myeloid-lymphoid (STRC-ML) lineage as well as long term repopulating cells (LTRC) into NOD/SCID/B2mnull immunodeficient mice at 3, 6, and 12 weeks post transplant. The involvement of CD26 in mediating any negative effects resulting from prior cytokine treatment will be directly assessed by treatment of the donor cells with CD26 inhibitors. Project Narrative Relevance: The purpose of this proposal is to explore a new direction in the field of stem cell transplantation by developing an understanding of the negative effects of cytokine usage on cord blood hematopoietic stem and progenitor cell trafficking, to determine whether these effects result mechanistically from an up-regulation of CD26, and to evaluate the potential novel use CD26 inhibitors to mitigate this effect. The clinical implications of this proposal are extremely high with respect to adult hematopoietic stem cell transplant patients who are currently unable to receive cord blood as a donor cell source due to limited numbers of stem cells obtainable from a single cord blood collection. We expect upon successful completion of the "proof of principle" style aims proposed here that a novel CD26 inhibitor based treatment strategy will emerge for use in combination with cytokine based therapies. Furthermore, we anticipate that with the appropriate subsequent research and clinical trials this research will have a positive impact on the survival of cord blood stem cell transplant patients.